In recent years, technology based on OLED (organic light-emitting diodes) has become established in the area of screen technology, so the first commercial products based on this are now available. In addition to screen technology, OLEDs are also suitable for use in two-dimensional lighting technology. For this reason, intensive research is being conducted on the development of new materials.
OLEDs are generally produced in layered structures consisting mainly of organic materials. For better understanding, a simplified structure is shown as an example in FIG. 1. The core of such components is the emitter layer, in which generally light-emitting molecules are embedded in a matrix. In this layer, negative charge carriers (electrons) and positive charge carriers (holes) meet and recombine into so-called excitons. The energy contained in the excitons can be released by the corresponding emitters in the form of light, and in this case the term electroluminescence is used. An overview of the function of OLEDs is available, for example, in H. Yersin, Top. Curr. Chem. 2004, 241, 1 and H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials,” Wiley-VCH, Weinheim, Germany, 2008.
Since the first reports on OLEDs (Tang et al., Appl. Phys. Lett. 1987, 51, 913) this technology has undergone continuous further development, especially in the area of emitter materials. Whereas the first material based on purely organic molecules were able to convert a maximum of 25% of the excitons into light because of spin statistics, the use of phosphorescent compounds made it possible to circumvent this fundamental problem, so that at least theoretically all excitons can be converted to light. These material are generally transition metal complexes, in which the metal is selected from the third period of the transition metals. Primarily very expensive noble metals such as iridium, platinum or gold are used. (Also see H. Yersin, Top. Curr. Chem. 2004, 241, 1 and M. A. Baldo, D. F. O'Brien, M. E. Thompson, S. R. Forrest, Phys. Rev. B 1999, 60, 14422). In addition to the expense, the stability of the materials is sometimes the stability of the materials presents drawback for their use.
A new generation of OLEDs based on the use of delayed fluorescence (TADF: thermally activated delayed fluorescence or singlet harvesting). In this process, for example, it is possible to use Cu(I) complexes which, because of a small energy distance between the lowest triplet state T1 and the singlet state S1 (ΔE(S1−T1) located above it, triplet excitons can thermally return to a singlet state. In addition to the use of transition metal complexes, purely organic molecules can also utilize this effect.
Some such TADF materials having already been used in the first optoelectronic components. The solutions to date, however, have drawbacks and problems. The TADF materials in the optoelectronic components often do not have sufficient long-term stability, thermal stability or chemical stability against water and oxygen. In addition, not all important emission colors are available. Furthermore, some TADF materials are not vaporizable and thus are not suitable for use in commercial optoelectronic components. Additionally, some TADF materials do not have suitable energy layers for the other materials used in the optoelectronic component (e.g., HOMO energies of TADF emitters of greater than or equal to −5.9 eV). Sufficiently high efficiencies of the optoelectronic components cannot be achieved at high power densities or high luminous densities with all TADF materials. In addition, the synthesis of some TADF materials is complicated.